echo

Echo,

in Greek mythology, mountain nymph. She assisted Zeus in one of his amorous adventures by distracting Hera with her chatter. For this Hera made her unable to speak except to repeat another's last words. She fell in love with Narcissus, but when he rejected her, she pined away until only her voice remained. In another myth, she was loved by Pan, who, because he could not win her, caused shepherds to tear her asunder; Gaea buried her limbs, leaving only her voice.

echo,

reflection of a sound wave back to its source in sufficient strength and with a sufficient time lag to be separately distinguished. If a sound wave returns within 1-10 sec, the human ear is incapable of distinguishing it from the orginal one. Thus, since the velocity of sound is c.344 m (1,130 ft) per sec at a normal room temperature of about 20&degC; (68&degF;), a reflecting wall must be more than 16.2 m (56 1-2 ft) from the sound source at this temperature for an echo to be heard by a person at the source. In this case the sound requires 1-20 sec to reach the reflecting surface and the same time to return. Bats navigate by listening for the echo of their high-frequency cry. Sonar and depth sounders work by analyzing electronically the echo time lag of sound waves, generally between 10 and 50 kilohertz, produced by underwater transducers. Radar sets broadcast radio waves, usually between 100 and 10,000 megahertz, pick up the portion reflected back by objects, and electronically determine the distance and direction of the objects. A sound echo that is reflected again and again from different surfaces, as by parallel walls in a tunnel, is called reverberation. When a surface reflects sound it partially absorbs and partially reflects the energy. As the process is repeated the sound becomes weaker and weaker and eventually ceases.

Echo

A sound wave which has been reflected or otherwise returned with sufficient magnitude and time delay to be perceived in some manner as a sound wave distinct from that directly transmitted. Multiple echo describes a succession of separately indistinguishable echos arising from a single source. When the reflected waves occur in rapid succession, the phenomenon is often termed a flutter echo. Echoes and flutter echoes are generally detrimental to the quality of the acoustics of rooms. They may be minimized through the proper selection of room dimensions, room shape, and distribution of sound-absorbing materials.

Echoes have been put to a variety of uses in measurement problems. For example, the distance between two points can be measured by timing the duration required for a direct sound originating at one location to strike an object at the other point and to return an echo to the location of the initial source. Ultrasonic echo techniques have achieved considerable success in nondestructive testing of materials. SeeReflection of sound, Sound, Ultrasonics

Echo

a device used in composition and performance that consists in the repetition of a musical phrase, gradually decreasing in volume, by the same or different voices or instruments. The echo is used mainly in choral works, operas, orchestral music, and chamber music. Sometimes the device is the basis for entire musical works or sections of works, as in O. di Lasso’s madrigal “O là, o che bon eccho,” or the section entitled “Echo” in J. S. Bach’s Partita in B minor for harpsichord. This device has been frequently used in operatic scenes (Orfeo ed Euridice by Gluck and Ariadne auf Naxos by R. Strauss), and in classical compositions {Echo, a composition for two-string trios by Haydn, Hoboken II, 39; Nocturne for Four Orchestras by Mozart, K. 286). One of the registers of the organ is also called an echo.

Echo

(from the name of the nymph Echo in Greek mythology), an acoustic, electromagnetic, or other wave reflected from some barrier and perceived by an observer. For example, an acoustic echo may be observed upon reflection of an acoustic pulse, such as a knock or a brief, abrupt shout, from surfaces with good reflecting properties. An echo is discernible to the ear if the transmitted and received pulses are separated by a time interval τ ≥ 50–60 msec.

An echo becomes multiple if there are several reflecting surfaces (near a group of buildings or in the mountains), the sound from which reaches the observer at moments of time separated by intervals of τ ≥ 50–60 msec. A harmonic echo arises upon the scattering of sound with a broad frequency spectrum by barriers with dimensions that are small compared with the wavelengths λ of the components of the spectrum. In an enclosure, numerous separate echoes merge into continuous repercussions (seeREVERBERATION).

An echo may be a means of measuring the distance r from a signal source to the reflecting object: r = cτ/2, where τ is the time interval between the signal burst and return of the echo and c is the velocity of wave propagation in the medium. Various applications of echo signals are based on this principle. The operation of sonar is based on the acoustic echo, which is also used in navigation, where echo sounders are used to measure depth. Electromagnetic echo is used in radar; being reflected from the ionosphere, it permits shortwave radio communication over great distances, as well as determination of the properties of the ionosphere. The echo-wave principle is also beginning to be used in the optical band of electromagnetic waves generated by a quantum optical generator. Elastic waves propagating in the crust of the earth, reflecting from layers of various rocks, form a seismic echo that is used to locate mineral deposits. Echoes can be used to measure the depth of boreholes (echo ranging of wells) and the height of the liquid level in tanks (ultrasonic level gauges). Echo methods are extensively used in ultrasonic flaw detection. For some animals, such as bats, dolphins, and whales, acoustic echo is a means of orientation and of locating prey.

A wave packet that has been reflected or otherwise returned with sufficient delay and magnitude to be perceived as a signal distinct from that directly transmitted.

echo

Sound waves which have been reflected to a listener with sufficient magnitude and time delay so as to be perceived separately from those communicated directly to the listener.

echo

i. In radar, a wave reflected from one or more points and returned with a magnitude and a time interval sufficient to be perceived and interpreted. It is observed on the radar indicator. See blip.ii. The pulse of reflected RF (radio frequency) energy.iii. A reflected sound wave.

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